U.S. patent number 5,062,991 [Application Number 07/532,434] was granted by the patent office on 1991-11-05 for in situ use of gelatin in the preparation of uniform ferrite particles.
This patent grant is currently assigned to Coulter Corporation. Invention is credited to Alexander Burshteyn, Olavi Siiman.
United States Patent |
5,062,991 |
Siiman , et al. |
November 5, 1991 |
**Please see images for:
( Certificate of Correction ) ** |
In situ use of gelatin in the preparation of uniform ferrite
particles
Abstract
A method is described for the preparation of uniform colloidal
particles of ferrites, containing manganese (II), zinc(II),
barium(II), iron(II), cobalt(II), or nickel(II), or a mixture of
manganese(II) and zinc(II), at a relatively low temperature in the
presence of a gelatin solution which acts as a support vehicle for
the nucleation and growth of colloidal particles of metal oxide and
for dispersion as separate single particles.
Inventors: |
Siiman; Olavi (Davie, FL),
Burshteyn; Alexander (Miami Lakes, FL) |
Assignee: |
Coulter Corporation (Hialeah,
FL)
|
Family
ID: |
24121790 |
Appl.
No.: |
07/532,434 |
Filed: |
June 4, 1990 |
Current U.S.
Class: |
516/101; 428/403;
427/213.35; 252/62.51R |
Current CPC
Class: |
B03C
1/01 (20130101); H01F 1/445 (20130101); H01F
1/061 (20130101); B01J 13/0013 (20130101); Y10T
428/2991 (20150115) |
Current International
Class: |
B03C
1/005 (20060101); B01J 13/00 (20060101); B03C
1/01 (20060101); H01F 1/032 (20060101); H01F
1/06 (20060101); H01F 1/44 (20060101); B01J
013/02 (); C09D 005/23 () |
Field of
Search: |
;252/62.51,62.53,62.54,313.1,309,315.2 ;428/402,403
;427/213.35 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
RBendaoud et al., Trans. On Magnetics, MAG-23, 3869-3873 (1987).
.
T. Sugimoto et al., J. Coll. Interface Sci., 74:227-243 (1980).
.
H. Tamura et al., J. Coll. Interface Sci., 90:100 (1982). .
A. Regazzoni et al., Corrosion, 38:212 (1982). .
A. Regazzoni et al., Colloids & Surfaces, 6:189 (1983). .
E. Matijevic, J. Coll. & Interface Sci., 117:593 (1987). .
P. H. Hess et al., J. App. Poly Sci., 10:1915-1927 (1966)..
|
Primary Examiner: Stoll; Robert L.
Assistant Examiner: Sweet; Greg M.
Attorney, Agent or Firm: Cass; Myron C.
Claims
I claim:
1. A method of making colloidal ferrite particles of uniform size
and shape comprising:
A. mixing a first solution of potassium nitrate and potassium
hydroxide or sodium nitrate and sodium hydroxide, respectively,
which has been nitrogen gas purged and a
B. second solution of a ferrous salt, divalent metal salt and a
gelatin solution which second solution has been nitrogen
purged;
C. sweeping the gelatinous metal hydroxide mixture of the two
solutions with nitrogen gas and ripening same to form a ferrite
hydrosol at a predetermined low temperature for a selected period
of time;
D. washing the hydrosol with said gelatin solution by magnetic
separation and redispersion,
whereby to form separate single metal ferrite particles coated with
gelatin.
2. The method of claim 1 in which the metal ion of said divalent
metal salt is the Fe2.sup.+ ion.
3. The method of claim 2 in which said ferrous salt can be either a
chloride or sulfate salt.
4. The method of claim 1 in which the metal ion of said divalent
metal salt is selected from the group consisting of Mr.sup.2+,
Co2.sup.+, Ni2.sup.+ and Zn2.sup.+ ions.
5. The method of claim 1 in which the metal ion of said divalent
metal salt is selected from the mixed group of Mn2.sup.+ and
Zn2.sup.+ ions.
6. The method of claim 4 in which said divalent metal salt is
selected from the group consisting of a metal chloride, metal
sulfate and metal nitrate salt.
7. The method of claim 5 in which said divalent metal salt is
selected from a mixture of metal chloride, metal sulfate and metal
nitrate salts.
8. The method of claim 6 in which said ferrous salt can be either a
chloride or sulfate salt.
9. The method of claim 1 in which the divalent metal salt is
selected from the group consisting of barium nitrate and barium
chloride and the ferrous salt comprises ferrous chloride.
10. A colloidal ferrite particle produced by the method of claims 1
or 2.
11. Colloidal particles which are monodispersed having a ferrite
structure and a mean particle diameter of approximately 0.1 to 1.0
microns, said particles including a divalent metal ion selected
from the group consisting of manganese(II), zinc(II), mixed
Mn(II)-Zn(II), iron(II), barium(II.), cobalt(II) and nickel(II) and
characterized as being well-defined and of uniform size and
shape.
12. The colloidal particles of claim 11 in which ferrite particles
of manganese(II), zinc(ID, mixed manganese(II)-zinc(II) and
barium(II) display a ferrimagnetic response to a magnetic
field.
13. The colloidal particles of claim 11 in which the ferrite
particles of iron(II), cobalt(II) and nickel(II) display a
ferromagnetic response to a magnetic field.
Description
FIELD OF THE INVENTION
This invention relates generally to a method for the preparation of
metal oxide particles. More specifically, this invention is
directed to an improved method for preparing uniform colloidal
ferrite particles containing manganese(II), zinc(II), mixed
manganese(II)-zinc(II), iron(II), barium(II), cobalt(II), or
nickel(II). The particles of this invention are formed at a
relatively low temperature in the presence of gelatin which acts as
a vehicle for their nucleation and growth and for their dispersion
into separate single particles of uniform size and shape.
BACKGROUND OF THE INVENTION
The invention is directed to overcoming the problem associated with
obtaining well-dispersed colloidal particles of uniform size and
shape of ferrites containing manganese(II), zinc(II), mixed
manganese(II)-zinc(II), iron (II), barium(II), cobalt(II), and
nickel(II). Colloidal particles of manganese(II) and zinc(II) or
mixed manganese(II)-zinc(II) ferrites of uniform shape and size
have not previously been reported. The invention provides a method
of preparation of magnetic metal oxide particles in the presence of
a polymer, solubilized in an aqueous medium wherein the colloidal
particles of ferrites containing manganese(II), zinc(II), mixed
manganese(II) -zinc(II), iron(II), barium(II), cobalt(II), and
nickel(II), are of a well-defined and uniform shape and size and
are dispersed as single particles in an aqueous media.
Ferrites containing manganese and/or zinc in fine particle form
represent an important class of ferromagnetic materials. Most
preparations of manganese ferrites have been carried out at high
temperatures (1000.degree.-2000.degree. C.) from solid solutions to
produce large crystallites [German Patent, DE 3619746 Al, Japanese
Patents, JP 8791423 A2 and JP8791424 A2]. Lower temperature
(350.degree. C.) decomposition of a mixed Mn-Fe oxalate, followed
by reduction with H.sub.2 /H.sub.2 O, gave a polycrystalline powder
which was characterized as a solid solution of Fe.sub.3 O.sub.4 and
MnFe.sub.2 O.sub.4. Low temperature methods assure crystallization
of manganese ferrite in the spinel structure as ferrimagnetic fine
particles [R. Bendaoud et.al., IEEE Trans. Magnetics, MAG-23:
3869-3873 (1987)].
Notable progress in obtaining monodispersed magnetite and ferrite
(Co.sup.2+, Ni.sup.2+) particles has been made [T. Sugimoto and E.
Matijevic, J. Coll. Interface Sci., 74: 227-243 (1980); H. Tamura
and E. MatiJevic, J. Coll. Interface Sci., 90: 100-109 (1982); A.
E. Regazzoni and E. Matijevic, Corrosion, 38: 212-218 (1982); A. E.
Regazzoni and E. Matijevic, Colloids Surf., 6: 189-201 (1983); E.
Matijevic, J. Coll. Interface Sci., 117: 593-595 (1987); X. J. Fan
and E. Matijevic, Patent Application WO 88/05337]. In every case,
however, the bulk of the magnetic particles in suspension is
irreversibly aggregated into large clusters that have a wide range
of sizes and shapes. Also, the hydrophobic surface of bare metal
oxide particles not only contributes to their agglomeration but
also makes them unsuitable for manipulation in aqueous solutions of
biological molecules, buffered near pH 7.
Some success in the preparation of polymer-magnetite composite
particles of uniform spherical shape has been achieved through the
emulsion polymerization of vinyl aromatic monomer in the presence
of ferrofluid seed particles which become embedded inside the
polymer latex [U.S. Pat. No. 4,358,388 and 4,783,336]. Control over
the size of the magnetic latex particles is poor, therefore,
resulting in particles with a wide range of sizes and magnetic
content. When external surface carboxylic acid groups are
introduced, the magnetic latex particles are hydrophilic to some
degree but still cannot be dispersed as single particles in
buffered aqueous media near pH 7. Coating of these particles by
covalent attachment of aminodextran has been carried out to give
the particles a hydrophilic shell. These aminodextran-coated
particles are stable in an aqueous buffer and have been covalently
linked with various monoclonal antibodies (IgG and IgM) for cell
depletion.
Uniform polymer-ferrite or -maghemite (magnetic hemalite) composite
particles have been prepared by crystallizing the magnetic oxide
inside uniform spherical and porous polymer particles
[International Patent Appliction WO83/03920; J Ughelstad et.al. in
"Microspheres: Medical and Biological Applications", Eds., A.
Rembaum and Z. A . Tokes, CRC Press, Inc., Boca Raton, FL, 1988].
Metal salts were diffused into the pores of the particle and
adjustment of pH or oxidation was carried out as required.
Alternatively, magnetic porous particles of metal oxide were first
prepared and then, the pores were filled and covered with
hydrophobic polymer. In both cases it was recognized that an
additional hydrophilic polymer coating was required for better
specific bead performance.
Solubilized polymers have been used to control the nucleation and
growth of various metal particles. The concept of nucleation of
metal particles in the domain of the polymer molecule was first
described in the formation of cobalt organosols by thermal
decomposition of dicobalt octacarbonyl in toluene and other organic
solvents with various solubilized polymers [P. H. Hess and P. H.
Parker, (Jr., J. Appl. Polym. Sci, 10: 1915-1927 (1966)]. The
classic protective agent for colloids is gelatin ["The Theory of
the Photographic Process", T. H. James, MacMillan Publ. of
polyacrylic acid and polyethyleneimine-N-alkylacetic acid have been
used to obtain stable hydrosols of gold, silver, copper, and
platinum metals [H. Thiele and H. S. von Levern, J. Coll. Sci., 20:
679-694 (1965)]. Colloida dispersions of very small rhodium,
iridium, osmium, palladium, platinum, silver, and gold particles in
ethanol or methanol with polyvinyl alcohol (PVA) or
polyvinylpyrrolidone (PVP) as stabilizer have been prepared [H.
Hirai, J. Macromol. Sci. Chem., A12: 1117-1141 (1978); O. Siiman
et.al., Chem. Phys. Lett., 100: 163-168 (1983); A. Lepp and O.
Siiman, J. Coll. Interface Sci., 105: 325-341 (1985); O. Siiman and
W. P. Hsu, J. Chem. Soc., Faraday Trans. 1, 82: 851-867 (1986)] .
Functional, soluble polymers have been used to control the
formation of colloidal dispersions of selenium and iron [T. W.
Smith and R. A. Cheatham, Macromolecules, 13: 1203-1207 (1980); T.
W. Smith and D. Wychick, J. Phys. Chem., 84: 1621-1629 (1980)].
Recently, hydroxypropyl cellulose was used in the formation and
stabilization of monodisperse TiO.sub.2 particles by hydrolysis of
titanium tetraethoxide in ethanol [J. H. Jean and T. A. Ring,
Colloids Surf., 29:273-291(1988)].
Monodispersed metal ferrite particles have several important
applications. They have magnetic properties that are useful for the
manufacture of transformers, inductors, audio and video recording
heads. Gelatin-coated MnFe.sub.2 O.sub.4 or ZnFe.sub.2 O.sub.4
particles with attached monoclonal antibody represents a completely
biodegradable magnetic separation system for biological cells. The
particles, coating, and any attached monoclonal antibody can be
phagocytosed without killing the cells. Also, gelatin, monoclonal
antibody, and cell(s) may be separated from the magnetic particles
by enzymatic cleavage of peptide bonds in gelatin, such as, by
using trypsin, papain, collagenase and other digestive enzymes. The
particles may, furthermore, be used as specific cell surface
markers. This invention provides for effective preparation of such
monodispersed metal ferrite particles.
SUMMARY OF THE INVENTION
In a method for the preparation of monodispersed colloidal
particles of ferrites of manganese, zinc, mixed manganese-zinc,
iron, barium, cobalt and nickel, an aqueous metal hydroxide gel is
first formed by mixing ferrous and other metal salts in an aqueous
gelatin solution with potassium or sodium hydroxide and potassium
or sodium nitrate solution, all purged with nitrogen gas. The
conversion of the gel to the metal oxide sol is achieved by mild
thermal treatment at 90.degree. C. (low temperature) for 4-72
hours, during which nitrate oxidation of ferrous iron occurs. This
incubation period also serves to degrade the gelatin as noted by
its lower viscosity. Only one type of gelatin, type B or
alkali-cured, with a pI range of 4.75 to 5.0 was found optimal for
in situ use.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a scanning electron micrograph of manganese ferrite
particles with white bar indicating a scale of 1 micron length.
FIG. 2 is a graph of mobility versus pH for bare manganese ferrite
particles in 1 mM aqueous sodium nitrate suspension at 25.degree.
C.
DESCRIPTION OF PREFERRED EMBODIMENT
Gelatin as a support vehicle for the formation of uniform metal
oxide particles has several important roles. First, it serves as a
buffer medium to neutralize acid, a product of the heat treatment.
Second, gelatin molecules act as loci for the supersaturation of
ferrite precursors and the formation and immobilization of ferrite
nuclei. They also act as domains to restrict subsequent growth of
nuclei and prevent the aggregation of particles. Various amino acid
residues (asp, glu, lys, his, met) of gelatin can provide
functional groups (carboxylate oxygen, amino nitrogen, imidazole
nitrogen, thioether sulfur) through which parts of the reactant
amorphous Fe(OH).sub.2 gel and its successors bind to gelatin. The
alkali-cured gelatin, that is most successful in promoting the
formation of single, uniform particles, contains an excess of
carboxylic acid residues, useful in attachment to iron in the
Fe(OH).sub.2 gel. Single particles of ferrites are then protected
from aggregation by steric repulsion between adsorbed gelatin
molecules. Gelatin can also adsorb to the hydrophobic surface of
the product, the uncharged metal oxide particles, through its
hydrocarbon residues, leaving its hydrophilic residues exposed to
the solvent. In subsequent stabilization and use of the magnetic
particles, gelatin usage allows chemical linkers to be used in
fixing gelatin around particles to produce a stable composite from
which gelatin can not be released by physical means. It also allows
covalent coupling of a monoclonal antibody, an enzyme, or other
proteins to the gelatin-coated particle. It is believed that no
other polymer has been successfully substituted for gelatin in its
aforementioned functions.
The choice of the metals for ferrite particle formation, involved
two principles. Firstly, ferrimagnetic or superparamagneti
particles were preferred over ferromagnetic ones in the size
ranges, 0.1 to 1.0 .mu.m in diameter. The former do not possess a
permanent magnetic moment but do become magnetized in the presence
of a magnetic field. In this way, aggregation possibilities created
by the alignment of particle moments are avoided. It is known that
ferrites, which have crystal structures of the normal spinel
structure type, are ferrimagnetic [A. F. Wells, Structural
Inorganic Chemistry, 5th ed., Clarendon Press, Oxford, 1984]. The
requirement for the M.sup.2+ metal ion to occupy tetrahedral sites
in a normal spinel structure is that it gives no crystal field
stabilization energy. Metal ion configurations d.sup.O (Sr.sup.2+,
Ba.sup.2+) high-spin d.sup.5 (Mn.sup.2+), and d.sup.10 (Zn.sup.2+),
satisfy this condition. Other metal ions (Fe.sup.2+, Co.sup.2+,
Ni.sup.2+) form inverse spinel structures in which M.sup.2+
occupies octahedral sites, Fe(MFe)O.sub.4, since these ions have
significant octahedral crystal field stabilization energies. The
designation "M" refers to a metal.
Secondly, the solubility of the M(OH).sub.2 species in water should
be greater than that of Fe(OH).sub.2. Solubility products of
representative metal hydroxides at 25.degree. C. are as follows:
2.04.times.10.sup.-13, Mn (OH).sub.2 ; 4.79.times.10.sup.-17, Fe
(OH).sub.2 ; 1.09.times.10.sup.-15, Co(OH).sub.2 ;
5.54.times.10.sup.-16, Ni(OH).sub.2 ; 7.68.times.10.sup.-17,
Zn(OH).sub.2 [Handbook of Chemistry and Physics, 64th ed., CRC
Press, Boca Raton, FL, 1984, p. B-219]. A value of
Ksp(M(OH).sub.2)>Ksp (Fe(OH).sub.2) is required so that some
M.sup.2+ will dissolve in aqueous solution and be able to diffuse
through the Fe(OH).sub.2 gel and substitute for some of the
FE.sup.2+ ions. Thus, the more soluble Mm(OH).sub.2, Co(OH).sub.2,
and NI(OH).sub.2 give uniform, submicron ferrite particles in our
preparative procedure. The cobalt and nickel ferrites showed
clustering of particles similar to that of ferromagnetic magnetite
particles after multiple washings and magnetic separations.
Ferrimagnetic manganese, zinc, and mixed manganese-zinc ferrites
showed little or no tendency to aggregate. Also, Zn(OH).sub.2 is
almost as insoluble as Fe(OH).sub.2 and Zn(OH).sub.2 is amphoteric,
so that stable zincate ions, ZnO.sub.2.sup.2 --are formed in basic
solution. Very small amounts of magnetic material were obtained by
this procedure with zinc. However, a 1:1 manganese-to-zinc sulfate
mixture gave uniform magnetic ferrite particles in good yieId. In
addition, Ba.sup.2+, which forms a very soluble hydroxide produced
uniform submicron magnetic ferrite particles.
The following solutions of reagent grade metal salts in double
distilled water (DDW) were prepared: 5M KOH, 2M KNO.sub.3, 1M
FeSO.sub.4, 1M MnSO.sub.4, 0.25M ZnSO.sub.4, 1M Co(NO.sub.3).sub.2,
1M Ni(NO.sub.3).sub.2, 0.1M Ba(NO.sub.3).sub.2, and 1M FeCl.sub.2.
All stock solutions except the KOH solution, were filtered through
0.2 .mu.m cellulose nitrate filters. The FeSO.sub.4 and FeCl.sub.2
solutions were purged with nitrogen gas for 10 minutes each time
they were used and not stored for more than one week. Gelatin, type
B, 225 Bloom, bovine skin, was prepared freshly as a 2% solution in
double distilled water and purged with nitrogen gas for 10
minutes.
EXAMPLE 1
PREPARATION OF MAGNETITE PARTICLES
10 mmol KNO.sub.3 (5mL) solution, 12.5 mmol KOH (2.5mL) solution,
and 11.25mL DDW were mixed and purged with N.sub.2 gas for 10
minutes (solution A). 6.25 mmol FeSO.sub.4 (6.25 mL) solution and
25 mL 2% gelatin solution was then added to solution A in a Pyrex
bottle, mixed, swept with N.sub.2 gas, capped tightly, and placed
undisturbed in oven at 90.degree. C. for 4 hours. After the
suspension of black magnetite particles had reached room
temperature, it was sonicated for 1/2 hour and the particles were
then washed 5 times 1% gelatin solution by magnetic separation and
redispersion in gelatin solution. The suspension was sonicated for
5 minutes between each wash.
Microscopic examination of the particles at
1000.times.magnification showed almost exclusively single,
spherical particles of about 0.5 .mu.m diameter. If the molar ratio
of Fe.sup.2+ OH.sup.-1 was changed from 1:2 to 1:1 by using 6.25
mmol KOH (1.25 mL) in the procedure, then similar black magnetite
particles aggregated into small clusters with some single particles
were obtained. When the KNO.sub.3 solution was pre-mixed with the
FeSO.sub.4 solution and other steps were unchanged, very small
reddish-brown particles in stringy aggregates were produced.
Therefore, the first procedure was adopted as the standard one in
subsequent experiments.
Other types of gelatin such as type B, 60 Bloom; type A, 175 Bloom;
and type A, 300 Bloom did not perform. More and larger aggregates
of magnetite particles were formed in each case. Various PVA
polymers in molecular weight (MW) range 3,000 to 106,000 gave
irregular large black aggregates of magnetite particles which could
not be dispersed. Similar results were observed with polyacry (5-6
.times.106, MW) and sodium dodecyl sulfate. Polyacrylic acid (2K
and 5K MW) and dextran (100K and 500K MW) gave large brown
crystallites which were only weakly magnetic; whereas, polyacrylic
acid and dextran of higher molecular weight gave no magnetic
material. Also, polystyrene sulfonic acid, PVP, and sulfonated
casing gave no magnetic material.
EXAMPLE 2
PREPARATION OF METAL FERRITES
In trials with other metals, namely, Mn.sup.2+, Zn.sup.2,
Co.sup.2+, Ni.sup.2+, and (M.sup.2+), the molar ratio of M2.sup.+
:Fe.sup.2+ was kept at 1:2 but nitrate instead of sulfate salts of
Co.sup.2+ and Ni.sup.2+ were used. The total metal-to-hydroxide
molar ratio was maintained at 1:2; but, the relative KNO.sub.3 to
total metal and KNO.sub.3 to KOH molar ratios were altered. In
preparing the mixed Mn-Zn ferrite, a 1:1 molar ratio of manganese
sulfate to zinc sulfate and the same total molar amount of
non-ferrous metal ions were used.
10 mmol KNO.sub.3 (5 mL) solution, 18.75 mmol KOH (3.75 mL), and
6.875 mL DDW were mixed and purged with N.sub.2 gas for 10 minutes
(solution C). 6.25 mmol FeSO.sub.4 (6.25 mL) solution, 3.125 mmol
Co(NO.sub.3).sub.2 (.sub.3.125 mL) solution, and 25 mL 2% gelatin
solution were mixed and purged with N.sub.2 gas for 10 minutes
(solution D). Solution D was added to solution C in a Pyrex bottle,
mixed, swept with N.sub.2 gas, capped tightly, and placed
undisturbed in an oven at 90.degree. C. for 5 hours. After the
suspension of brown particles had reached room temperature, it was
sonicated for 1/2 hour and the particles were then washed
5.times.with 1% gelatin solution by magnetic separation and
redispersion in gelatin solution. The suspension was sonicated for
5 minutes between each wash.
Cobalt and nickel ferrite particles of about 0.1 and 0.2 .mu.m in
diameter and of spherical shape were formed in large, loosely-held
brown aggregates. Zinc gave low yields of light brown magnetic
material (<0.2 .mu.m diameter) even after 72 hours of heat
treatment. Dark brown manganese ferrite particles of uniform,
spherical shape and 0.3 .mu.m diameter were obtained as single
particles in 83-88% yields. Similar light brown manganese-zinc
ferrite particles were produced in 49-55% yield after 72 hours of
heat treatment at 90.degree. C. For barium, the procedure had to be
modified since BaSO.sub.4 is insoluble in water. Thus, 6.25 mmol
FeCl.sub.2 (6.25 mL) solution, 0.5 mmol Ba(NO.sub.3).sub.2 (5.0 mL)
solution, and 25 mL 2% gelatin solution were mixed and purged with
N.sub.2 gas for 10 minutes (solution D). Solution C and the
remainder of the ferrite preparation procedure was unchanged except
10 mmol KOH solution (2 mL) was used and the heat treatment was
continued for 20 hours. Black barium ferrite particles of uniform
non-spherical shape with a 0.2 .mu.m diameter were produced.
Because of their favorable magnetic, size, and shape properties,
manganese ferrite particles were also prepared at larger scales and
analyzed further by physical means. Concentrations of reactants
were scaled up linearly at 250 and 500 mL total levels. For the 250
mL-scale, the heat treatment at 90.degree. C. was still for 5
hours, but it was increased to 48 hr to achieve better gel-to-sol
conversion on the 500 mL scale. Percentage yields based on a 2:1
molar ratio of FeSO.sub.4 : MnFe.sub.2 O.sub.4 were 83% at the 250
mL scale 204 were and 84% at the 500 mL scale. The particles were
washed exhaustively with DDW and then dried at 110.degree. C. and
weighed to constant weight. Elemental analyses were obtained on a
250 mL scale preparation as follows: Calculated for MnFe.sub.2
O.sub.4 : Mn, 23.82%; Fe, 48.43%; observed: Mn, 20.01%; Fe, 49.99%.
Duplicate pycnometer measurements of density for manganese ferrite
particles by displacement of DDW gave 4.24 and 4.23 g/cc. A
scanning electron micrograph (FIG. 1) of manganese ferrite
particles showed particles of spherical shape and uniform size. The
mean diameter for 414 particles was 0.29(0.08).mu.m. The specific
surface (S.sub.w) for manganese ferrite particles is then 4.89
m.sup.2 /g. This compares favorably with magnetite embedded
polystyrene latex beads as follows: (1) 0.7 .mu.m, 41% magnetite,
1.56 g/cc gives S.sub.w =5.50 m.sup.2 /g; (2) 0.98 .mu.m, 23%
magnetite, 1.28 g/cc gives 4.78 m.sup.2 /g. The most recent porous
ferrite hydrophobic polymer-filled and covered beads give an
S.sub.w range of 3-5 m.sup.2 /g. The electrophoretic mobility of
bare manganese ferrite particles in 1 mM aqueous nitrate at
25.degree. C., measured as a function of pH (adjusted with aqueous
sodium hydroxide or nitric acid) on the Coulter DELSA 440, is shown
in FIG. 2. An isoelectric point of about 3.7 for the colloidal
particles and a zeta potential of -65 mV at pH7 was obtained.
Elemental analyses were also obtained for manganese-zinc ferrite:
calculated for Mn.sub.0.5 ZN.sub.0.5 Fe.sub.2 O.sub.4 : Mn, 11.65%
Zn, 13.86%; Fe, 47.36%; observed: Mn, 10.86%; Zn, 11.61%; Fe,
47.12%. The density of manganese-zinc ferrite particles was 4.13
and 4.20 g/cc in duplicate measurements. The specific surface for
manganese-zinc ferrite particles is 4.97 m.sup.2 /g.
The method embodying the invention contemplates substituting a
mixture of sodium hydroxide and sodium nitrate for the potassium
hydroxide and potassium nitrate mixture. Also, divalent metal
nitrates, i.e., of Co.sup.2+, NiZ.sup.+, ZnZ.sup.+ and Ba2.sup.+
can be replaced by divalent metal chlorides and divalent metal
sulfates except for the divalent metal Ba2.sup.+ which will form an
insoluble barium sulfate compound. The divalent metal sulfates of
Mn.sup.2+, Zn.sup.2+, and Fe2.sup.+ can be replaced by divalent
metal chlorides and nitrates except for ferrous nitrate which is
unstable because of subsequent oxidation to form ferric
nitrate.
The relatively low temperature employed in practicing the invention
can vary in the range of 85.degree.-95.degree. C. Also, the time
period for nitrogen gas purging may be varied within appropriate
limits without adversely affecting practicing the invention. Also,
the percentage rating of the gelatin solution may vary within the
approximate range of 0.8% to 2.0%.
The colloidal particles prepared by the herein invention are
monodisperse with a ferrite structure and a mean particle diameter
of approximately 0.1 to 1.0 microns. The Mn(II) ferrite particles
contain approximately 17 to 21 percent by weight of manganous ions.
The Mn(II), Zn(II), mixed Mn(II)-Zn(II) and Ba(II) ferrite
particles have a ferrimagnetic response to a magnetic field, i.e.,
possess no magnetic memory, while magnetite and Co(II) and Ni(II)
ferrite particles have a ferromagnetic response.
The Mn(II) and mixed Mn(II)-Zn(II) particles have a significantly
lower density of 4.2 g/cc than 2 g/cc density of the magnetite
particles whereby submicron particles are made more buoyant in
aqueous suspensions.
The Mn(II), Zn(II) and mixed Mn(II)-Zn(II) particles have surfaces
which are less hydrophobic than the surfaces of magnetite particles
which have an isoelectric point of 6.7 as compared to the
isoelectric point of 3.7 of Mn(II) ferrite particles. The Mn(II)
ferrite particles are more stable in aqueous suspension in
proximity to pH7 due to electrostatic repulsion between negatively
charged particles. The Mn(II) and mixed Mn(II)-Zn(II) ferrite
particles have a high specific surface of 5m.sup.2 /g.
* * * * *